I used to drink a pot a day when I was younger. Now, it's 3 or 4 cups max and never any after 5:00. Otherwise I would be up half the night. I can't bring myself to drink decaf. My philosophy is, "what's the point" when it comes to fake coffee.

As far as "how much is too much", I think it depends on what type of coffee you're drinking - caffeinated versus decaf - along with what you add to it. If you're drinking caffeinated coffee and adding lots of sugar and fattening or sugar-laden creamer, in my opinion, less is better. Personally, I limit myself to a half pot of decaf coffee per day, sweetening with sugar-substitute and sugar-free creamer. If I drink caffeinated coffee, which is rare, I limit myself to just a couple of cups.

How big are the mugs, and is he drinking any water in between? It sounds like he is at the point where caffeine isn't doing much for him anymore, and so he has to keep drinking more and more just to generate any buzz. I think as long as he isn't living on coffee alone, five mugs is still within a safe limit.

How much is too much? If you felt that your body is not in a good condition if you drank coffee. For me, 5 mugs of coffee in a day are too much, especially if that mug is big. But I think if your boss are enjoying drinking 5 mugs coffee a day, I think he/she knows what he/she is doing and his/her limitations.